Immune-mediated infiammatory diseases are a major public health issue. In the lungs, these can be triggered by exposure to environmental anfigens and toxins, or by infection, and can result in severe respiratory disease and death. Defining the immunoregulatory mechanisms that normally function to prevent pulmonary inflammafion is therefore key to understanding the efiology of these diseases, and for developing therapeutic strategies to boost these acfivities in pafients. Regulatory T cells (TR) expressing the transcripfion factor Foxp3 play a critical role in prevenfing autoimmunity and limifing immune-mediated inflammafion. We have shown that during type-1 inflammatory responses, Foxp3+ TR upregulate the Thlspecifying transcripfion factor Tbx21 (T-bet), and that T-bet expression is crifical for proper TR homeostasis and funcfion during Thi-mediated inflammation. Therefore, the goals of this proposal are to determine in detail how loss of T-bet specifically within Foxp3+ TR impacts the initiafion, progression and terminafion of Thi responses in models of acute and persistent lung infection in vivo (Specific Aim 1), define the cytokines and cellular signals that direct TR expression of T-bet (Specific Aim 2), and analyze at the molecular level how FoxpS and T-bet combine to control the expression of genes involved in JhlfTR differenfiafion, homeostasis and funcfion (Specific Aim 3). Together, these experiments will generate an unprecedented understanding of the molecular specializafion of TR subsets during type-1 inflammafion, and provide a new framework in which to understand how so-called 'master transcription factors'direct the funcfional differenfiation of CD4+ T cell subsets.